Australasian Science Magazine

Zakharova_Natalia / iStockphoto

By Anna Kuparinen, Asta Audzijonyte & Elizabeth Fulton

Fish are becoming smaller all over the world as oceans change and catches increase, with even small changes having great consequences for ecosystems and fisheries.

Humans are affecting oceans all around the world – through fishing, pollution and climate change. Overfishing has become a particularly burning issue over the past few decades, as many commercially exploited fish stocks have declined to historically low levels.

In addition, a new problem seems to be emerging – fish around the world are getting smaller. One reason for this decrease in size is the fishing itself, as large, usually older fish are being caught and only young and small fish are left.

However, it goes deeper than that. If we take a fish today and compare it with a fish of the same age from the past, the fish of today are often smaller than they used to be a century ago. And again, it is fishing that appears to be the main culprit.

Fishing is typically size-selective, and it is usually the largest individuals that are being targeted. Rules around mesh sizes and minimum landing sizes mean that the largest fish are caught and kept. These regulations aim to protect young fish and allow them to grow and reproduce at least once before they are caught.

While the intent seems logical, there are unintended consequences of this policy. Fishing in this way may drive evolution towards smaller body size and earlier reproduction. The situation is equivalent to animal breeding except that it works in reverse – animal breeders leave the most valuable stock for reproduction, whereas fishing takes them out.

While observations of declining fish body sizes are abundant, scientists have had difficulties finding strong evidence that it is really a case of evolution influenced by fishing. This is because body size in fish is determined by many factors. There is certainly a genetic component, but also environmental ones such as water temperature, food supply, population density of fish and so on. These impacts all work at the same time, and sometimes in opposite directions.

Thus it is extremely difficult to exclude all possible environmental factors from wild fish stocks and be sure that changes in size are caused by genetic change. A number of research groups around the world are currently conducting extensive genetic analyses of affected fish stocks, aiming to find genes that are responsible for growth and find evidence for historical changes in these growth-related genes.

Even if direct genetic evidence may still be forthcoming, a growing volume of indirect evidence suggests that historical decreases in fish body sizes should be taken seriously. For example, if these changes are indeed at least partly caused by genes, the trend towards slower growth or early reproduction will be hard to reverse, even if fishing is completely stopped.

Climate change may also lead to changes in fish body sizes. For fish in cold waters, the warming of ocean temperatures is likely to increase growth. However, this initial increase will be reversed when water warms up even more and reaches the maximum temperature limit tolerated by the fish. Observations are already showing that increasing water temperatures are slowing the growth of fish in tropical waters. Warm water holds less oxygen than cold water, which means that fish have less oxygen to breathe and use for growth. Some modelling studies predict that by 2050 the average fish size will be up to 20% smaller than nowadays.

Regardless of whether fish are getting smaller because of climate change or evolutionary responses to fishing, such changes in fish phenotypes may not be beneficial for the fish or the fishers. On the one hand, fishing-induced evolution may help fish to survive under high fishing pressure. If this evolution did not exist, many intensively fished stocks might have collapsed by now.

While this evolutionary change may help fish to survive complete extinction, it does not necessarily allow for the recovery of stocks as we know them. This is because when a fish starts reproducing earlier and at a smaller size it will produce far fewer offspring than a large fish would. This means that there is less potential for the stock to grow, so the stock may remain at low levels and fisheries may remain unprofitable even if fishing pressure is greatly reduced.

But there might be an even worse scenario. Predation is typically strongly dependent on size. Predators can only eat prey up to a certain size, and the longer time a fish remains small, the more likely it will be eaten. In our research, recently published in Biology Letters, we investigated how declines in fish body size affected the fish stocks and fisheries using the south-eastern Australian marine ecosystem as a model.

A central tool in this research work was the Atlantis ecosystem modelling framework, a computer-based model developed by CSIRO that describes marine habitat and food web dynamics and predator–prey interactions from the smallest organisms such as plankton to the largest sharks and whales. Being a cutting-edge modelling tool for marine ecosystems, Atlantis is used to support research into fisheries and marine climate change and inform management.

We simulated a small decrease in size-at-age of five commercially important south-east Australian fisheries species: jackass morwong (Nemadactylus macropterus), tiger flathead (Platycephalus richardsoni), silver warehou (Seriolella punctata), blue grenadier (Macruronus novaezelandiae) and pink ling (Genypterus blacodes). In our ecosystem model, every year these five fish species grew just a bit slower, which meant that every year their average size was about 0.1% smaller. Over 50 years these changes meant there was an accumulated decrease in average size-at-age of about 4%.

We then compared how the predicted ecosystem dynamics of a system populated with smaller fish differed from simulations where fish growth remained stable over time. The results show that body size changes could have a fairly large impact on the fish.

Declines in fish body size typically led to substantial increases in the rates of predation mortality they experienced. A gradual but continuous decrease in the size of some fish triggered changes in feeding interactions that were reinforced every year.

For instance, as flathead became smaller they ate fewer young barracouta. After a few years there were more older barracouta, and these could eat more flathead than before. Therefore there were even less flathead, and they could eat even fewer young barracouta. A few years later there were even more older barracouta eating young flathead. This feedback loop meant that for some fish just a 4% decrease in average size increased their predation mortality by as much as 20–50%.

The predicted increase in predation mortality due to declining fish body size has substantial practical consequences for fisheries. Because more fish die as a result of predation there will be fewer fish caught in fisheries. This means that either fishing rates should be reduced or the fish stock will not sustain fishing for very many generations without collapsing. In extreme cases, predation mortality can increase so much that the fish stock may not persist even in the absence of fishing.

This appears to be what is happening for Atlantic cod in the Gulf of St Lawrence (north-west Atlantic). After the cod stock collapsed as a result of overfishing, predation of cod by grey seals increased. The cod stock is predicted to become extinct even though there is practically no cod fishing in the area.

These results stress the importance of monitoring and carefully considering even small changes in fish body size. Overfishing and the decline of commercially important fish stocks are global problems for which there are no simple solutions.

However, the use of best available scientific knowledge and modern methods to predict future development of fish stocks, coupled with a high level of precaution, are the keys to reversing declining trends in fish stock sizes and fisheries catches.